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CHAPTER 21:Ethernet MAC
600 S6J3200 Series Hardware Manual Document Number: 002-04852 Rev. *G
3.6.
IEEE Std 1588 and IEEE Std 802.1AS Support
IEEE Std 1588 is a standard for precision time synchronization in local area networks. It works with the
exchange of special Precision Time Protocol (PTP) frames. The PTP messages can be transported over
IEEE Std 802.3/Ethernet, over Internet Protocol Version 4 (IPv4) or over Internet Protocol Version 6
(IPv6) as described in the annex of IEEE Std 1588-2008.
Most IEEE Std 1588 functionality can be implemented in software but for greatest accuracy hardware
assist is required to detect when PTP event messages pass the MII interface (clock timestamp point).
Synchronization between master and slave clocks is a two stage process.
First the offset between the master and slave clocks is corrected by the master sending a Sync frame to
the slave with a follow up frame containing the exact time the Sync frame was sent. Hardware assist
modules at the master and slave side detect exactly when the Sync frame was sent by the master and
received by the slave. The slave then corrects its clock to match the master clock.
Second the transmission delay between the master and slave is corrected. The slave sends a delay
request frame to the master which sends a delay response frame in reply. Hardware assist modules at the
master and slave side detect exactly when the delay request frame was sent by the slave and received by
the master. The slave will now have enough information to adjust its clock to account for delay. For
example if the slave was assuming zero delay the actual delay will be half the difference between
transmit and receive time of the delay request frame (assuming equal transmit and receive times)
because the slave clock will be lagging the master clock by the delay time already.
For hardware assist it is necessary to timestamp when Sync and Delay_Req messages are sent and
received. The timestamp is taken when the message timestamp point passes the clock timestamp point.
For Ethernet the message timestamp point is the SFD and the clock timestamp point is MII interface. (The
IEEE Std 1588 spec refers to Sync and Delay_Req messages as event messages as these require time
stamping. Follow up, delay response and management messages do not require time stamping and are
referred to as general messages.)
IEEE Std 1588 version 2 (IEEE Std 1588-2008) defines two additional PTP event messages. These are
the peer delay request (Pdelay_Req) and peer delay response (Pdelay_Resp) messages. These
messages are used to calculate the delay on a link. Nodes at both ends of a link send both types of
frames (regardless of whether they contain a master or slave clock). The Pdelay_Resp massage contains
the time at which a Pdelay_Req was received and is itself an event message. The time at which a
Pdelay_Resp message is received is returned in a Pdelay_Resp_Follow_Up message.
IEEE Std 1588 version 2 introduces transparent clocks of which there are two kinds, peer-to-peer (P2P)
and end-to-end (E2E). Transparent clocks measure the transit time of event messages through a bridge
and amend a correction field within the message to allow for the transit time. P2P transparent clocks
additionally correct for the delay in the receive path of the link using the information gathered from the
peer delay frames. With P2P transparent clocks Delay_Req messages are not used to measure link delay.
This simplifies the protocol and makes larger systems more stable.
The Ethernet MAC recognizes ten different encapsulations for PTP event messages:
1. IEEE Std 1588 version 1 (UDP/IPv4 multicast)
2. IEEE Std 1588 version 1 (UDP/IPv4 multicast with VLAN)
3. IEEE Std 1588 version 2 (UDP/IPv4 multicast)
4. IEEE Std 1588 version 2 (UDP/IPv4 multicast with VLAN)
5. IEEE Std 1588 version 2 (UDP/IPv4 unicast)
6. IEEE Std 1588 version 2 (UDP/IPv4 unicast with VLAN)
7. IEEE Std 1588 version 2 (UDP/IPv6 multicast)
8. IEEE Std 1588 version 2 (UDP/IPv6 multicast with VLAN)